1. Field of the Invention
This invention relates to a novel single step method and apparatus for the manufacture of precision shaped hollow and/or solid ingots of various materials. Materials which are desirably formed into ingots include various ceramic and refractory materials, etc. The present invention has proven particularly useful in the production of fused quartz and fused silica ingots. The terms "fused quartz" and "fused silica," are today recognized within industry and science as relating to materials consisting primarily of amorphous SiO.sub.2. Such materials are successfully used for a variety of applications molded in the form of tubing, rod, fibers, plates or block material due to their combination of unique properties such as high chemical purity, high temperature and temperature shock resistance and due to their ability to conduct or transmit light waves ranging from ultraviolet to infrared.
2. Prior Art
In the German Herraeus process,.sup.1 ingots (solid only) of comparatively small diameter are produced by spraying fine crystalline quartz powder onto the cylindrical end face of a horizontally disposed, rotating fused quartz holder. However, the outer surface obtained by this prior art process is rough and requires substantial additional rework. In order to produce a useful ingot the lance obtained by this end spray method must be first washed in Hydrofluoric Acid, washed in distilled water and then be dried. Thereafter, it must be reheated in a separate furnace and the diameter is increased by accummulating the material in a larger diameter mold of graphite. Simultaneously, a graphite rod is employed to pierce the hole through the center in order to obtain a hollow ingot. In the German process the surfaces of the ingot will be contaminated with reaction products from the SiO.sub.2 and the carbon. Most detrimental is the incorporation of Silicon Carbide SiC. Subsequently, the impurities must be eliminated through mechanically grinding both the outer and the inner surfaces. After this, the ingot is not yet completed, since the surface contamination from the grinding processes, still requires the ingot to be washed again in Hydrofluoric Acid. In doing so, the accurately machined surfaces are often attached by the acid in an unpredictable manner thus destroying the effect of machining. Still another drawback of this method resides in the quartz powder losses resulting from the operation of the flame spray burner which is complex and combines the transportation of the powder with heating. FNT .sup.1 Malvery J. Gross, "Fused Quartz Manufacture in Germany," Fiat Final Report No. 536, Office of Military Government of Germany (U.S.), pp. 12-21.
In another method, described in U.S. Pat. No. 3,486,870, issued to Vervaart et al Dec. 30, 1969, employs an expensive rotating molybdenum or tungsten mandrel. The mandrel which is vertically arranged and serves as the starting form for growing hollow fused quartz ingot, is covered with a fused quartz tube and protected with a sheath of inert gases. The disadvantage of this method is that an expensive fused Quartz tube must be fitted over the ingot. Another disadvantage of this method is that frequent metal contamination occurs as a result of variations in the flow of protective gases and the temperature. Furthermore, the fact that the metallic mandrel is heated to very high temperatures while in contact with the quartz glass, increases the risk of destruction of the mandrel (by oxidation) beyond repair if a failure in the supply of protective gases occurs. While this older method produces hollow ingots with the above described limitations, provided with bores of accurate diameter, there is little control of the outer diameter. Still another disadvantage of this prior art spray method is that the ingot is started on a quartz glass disc and a quartz glass tube, their weight being added to the melt, thus representing a cost of manufacture which is lost each time such an ingot is produced.